1. Field of the Invention
The present invention relates to a phase detection method and a phase detecting apparatus capable of detecting phases of a plurality of phase signals on a real time basis, and also to a control method and a synchronous motor controller for performing sensorless synchronous control of the synchronous motor using the same.
2. Description of the Related Art
A typical synchronous motor controller includes an alternating current detector that detects electric current of a motor provided in one of feed lines from an inverter circuit to the motor, a motor-current-phase detector that detects a motor current phase on the basis of a motor voltage phase at the time of zero crossing of the current, and a calculating unit that calculates a voltage command or frequency command of the inverter circuit so that the motor current phase is same as a desired motor current phase. The inverter circuit is then controlled based on the calculation result. A conventional synchronous motor controller has been disclosed in Japanese Patent Application Laid-open No. H5-236789.
Moreover, technique of sensorless synchronous control of the synchronous motor is available. For example, in the method disclosed in Japanese Patent Application Laid-open No. 2006-223085, the position of a rotor is estimated based on a voltage equation for the motor. In this method, however, a highly accurate motor constant is required, and complicated control needs to be performed.
In the technique disclosed in Japanese Patent Application Laid-open No. H5-236789, a phase difference between the motor current phase and the motor voltage phase is detected at the time of zero-crossing of current, i.e., phase detection is performed for every 180°. Phase detection for every 180°, however, leads to poor detection accuracy, moreover it is not possible to detect instantaneous phase.
Generally, when the phase difference is detected on the real time basis, a three-phase signal is converted to a two-phase signal to obtain a vector phase. For example, when three-phase to two-phase conversion is performed with respect to the three-phase signal shown in the upper part of FIG. 1, waveforms of a real axis component and an imaginary axis component are obtained as shown in the upper part of FIG. 4. When arctan transform is performed using the real axis component and the imaginary axis component, as shown in the lower part of FIG. 4, a vector phase is obtained. In this conventional method, however, processing to obtain the detection result is complicated, and when phase detection is performed on the real time basis, a device having a large computing capacity is required.
Japanese Patent Application Laid-open No. 2004-336876 discloses a technique with which it is possible to detect the phase at an arbitrary timing. Even this method is a method for detecting discrete instantaneous phase, and therefore the detection accuracy is poor and phase detection cannot be performed on the real time basis.